The effects of close interbreeding on animals,
judging again from plants, would be deterioration in general vigour,
including fertility, with no necessary loss of excellence of form; and
this seems to be the usual result.
It is a common practice with horticulturists to obtain seeds from
another place having a very different soil, so as to avoid raising
plants for a long succession of generations under the same conditions;
but with all the species which freely intercross by aid of insects or
the wind, it would be an incomparably better plan to obtain seeds of the
required variety, which had been raised for some generations under as
different conditions as possible, and sow them in alternate rows with
seeds matured in the old garden. The two stocks would then intercross,
with a thorough blending of their whole organisations, and with no loss
of purity to the variety; and this would yield far more favourable
results than a mere exchange of seeds. We have seen in my experiments
how wonderfully the offspring profited in height, weight, hardiness, and
fertility, by crosses of this kind. For instance, plants of Ipomoea thus
crossed were to the intercrossed plants of the same stock, with which
they grew in competition, as 100 to 78 in height, and as 100 to 51 in
fertility; and plants of Eschscholtzia similarly compared were as 100 to
45 in fertility. In comparison with self-fertilised plants the results
are still more striking; thus cabbages derived from a cross with a fresh
stock were to the self-fertilised as 100 to 22 in weight.
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